Pipelines are commonly used to transport fluids both underground and subsea. It is typical for pipelines to be provided with some type of protective coating for anti-corrosion and/or mechanical purposes. Common types of coating include coal tar and asphalt enamel. A concrete coating or sheath can be applied to individual lengths of pipe in a coating yard, before the pipes are shipped to the location where the pipeline is to be assembled. Alternatively, in the case of underground pipelines, the coating can be applied to the pipe "over the ditch" after it is welded and immediately before it is placed in the ground. Yard-applied coatings are preferred in many instances because a superior coating job can be performed under the controlled conditions of a coating plant. Further, an outer coating, such as a concrete or plastic coating, can provide protection for insulation material that may be included between the pipe and the outer coating. Yard-applied coatings are also preferred when an inner pipe is used within a second, outer pipe. This allows the inner pipe to be coated when it is more accessible, before being placed within the second pipe. One example of the use of a pipe within a pipe is placement of a bore pipe at a road crossing.
Yard-applied coatings are also preferred when more than one pipeline is to be joined in a bundle. This allows each pipe to be coated before it is placed alongside another pipe. Pipeline bundles are useful in both underground and subsea applications, particularly for transporting separate fluid streams. For example, a pipeline bundle may be used to separate an export line from a production line so as to facilitate separate metering of the export and production fluids and accommodate different tax rates. Similarly, pipeline bundles may be used to separate a gas line from a liquid line, or a water line from a hydrocarbon line. Pipeline bundles may include pipelines of different diameters. For example, a smaller pipeline may be included with a larger pipeline in a "piggyback" arrangement. Pipeline bundles may be assembled from preformed pipelines. Alternatively, pipeline bundles may be formed simultaneously as the individual pipelines are formed from pipe segments. Assembly may occur in the yard or in the field, such as on a lay barge, and the pipeline bundle may be reeled for later use.
A variety of problems exist with the use of pipe having a yard-applied coating. These include problems related to the use of insulated pipe segments that have received a yard-applied coating, the use of pipelines having gaps in the yard-applied coating in the vicinity of weld joints between pipe segments, and the use of a combination of more than one pipeline having a yard-applied coating. Such combinations include the arrangement of a inner pipeline within an outer pipeline of larger diameter; as well as pipeline bundles, in which pipelines are arranged adjacently.
With regard to insulated pipe, a problem with yard-applied coatings is the need to cover an exposed end of insulation. A wrap sealing an exposed end of insulation is commonly called a "water stop". Insulation under a plastic coating typically does not extend beyond the coating into the gap region to either side of the weld joint, thus leaving a portion of the insulation exposed. Exposed insulation is vulnerable to water seepage, which is undesirable, as it contributes to deterioration of the thermal insulation. One technique for providing a water stop is to cover the exposed area of insulation as well as the area of coating and pipe to either side with a shrink sleeve. The shrink sleeve is formed of material that shrinks upon the application of heat, causing it to fit tightly over the region of exposed insulation. A disadvantage of this technique is its dependence on adhesive to bond the sleeve to the pipe, forming a seal. The adhesive is susceptible to deterioration, so that the bond may become unreliable.
With regard to pipe cathodically protected by a ribbon anode, a common problem with yard-applied coatings is the need to mechanically hold the anode to the pipe to prevent sagging of the anode away from the pipe when it is handled. To aid in the corrosion protection of metal pipe, particularly in subsea applications, a sacrificial anode is commonly anchored to the coated pipe and connected electrically to the pipe metal. An anode may take a variety of forms, a common form being that of a length of ribbon anode. More than one anode may be placed at intervals along the pipeline. In the case of pipeline bundles, one anode may be used to protect adjacent pipes. An anode is typically connected electrically to the pipe(s) at each end of the anode. A region of the plastic coating is removed to expose a portion of pipe, the anode is welded to the pipe in this region, and the weld area covered with epoxy for protection. Typically the weld is a thermite weld, such as is known under the tradename of CAD weld. An alternate weld is the pin weld, which penetrates further into the pipe. In either case, the welds at either end of the anode are insufficient to maintain the length of the anode close to the pipeline and prevent the anode from sagging away from the pipeline, for example when the pipeline flexes during handling.
With regard to the joint area of a pipeline, it is necessary to coat the uncoated portion of the pipe on either side of the weld joint once two lengths of coated pipe have been joined together. This portion is typically less than about three feet long. There are two known techniques for coating the weld joint area in the field. The first entails wrapping the weld joint area with a protective tape that overlaps the yard-applied coating at either end. The protective tape is not welded or fused to the yard-applied coating and therefore depends on adhesive to bond it to the yard-applied coating. One of the primary disadvantages of this technique is that it is not possible to achieve a hermetic seal of great integrity. If tape is utilized in the area of the weld joint, the seal in that area will not be of the same quality as the seal from the yard-applied coating on the remainder of the pipe, due in part to deterioration of exposed adhesive near the tape edges.
Another common technique for coating the area of the weld joint area of pipe in the field is to utilize a field mold that is fitted around the pipe. The mold forms an annular space around the weld joint, into which space is poured or injected a coating material that is compatible with the coating on the remainder of the pipe. While this process does result in the seal to the pipe surface in the area of the weld joint being of generally the same integrity as the coating on the remainder of the pipe, it lacks good adhesion to the plant-applied coating at the interface, as it may be difficult to achieve a good bond between the plant-applied coating and the field-applied coating. In addition, the process is relatively time consuming and therefore expensive.
In recent years, new types of pipe coatings have been developed utilizing extruded thermoplastic materials. Coatings of this type provide a hermetic seal of high integrity along the length of the pipe but are not well suited to the techniques described above for coating the weld joint areas of a pipeline in the field.
With regard to a pipeline within another pipeline, it is necessary to support a pipe placed within another pipe of a different diameter such that the inner pipe does not contact the outer pipe and is thus electrically isolated from the outer pipe. Without support for the inner pipe, gravity would cause the inner pipe to rest on the outer pipe, which is undesirable because it disrupts cathodic protection of each pipe by electrically shorting them out against each other. Such a support is commonly called a spacer or a casing insulator. Mechanical spacers may be placed in the annular space between the pipes to support the inner pipe. For example, when a bore pipe is used at a road crossing, one or more spacers are placed at a distance from each other longitudinally along the pipe. One technique that is known for separating the inner pipe from the outer pipe is the use of runners, made for example out of epoxy, which are arrayed circumferentially around the pipe and held to the pipe, for example with adhesive, clamps, or mechanical strapping. This technique has the disadvantage that the spacer(s), may slip along the inner pipe when the pipe combination is handled, causing the spacers to bunch or otherwise deviate from a beneficial placement.
With regard to a pipeline bundle, it is necessary when forming a pipe bundle to provide an apparatus for holding the pipes in the bundle together. There are two known techniques for joining pipes in a pipe bundle. In the first technique, pipe bundles are held together by wrapping a metal strap around the outer part of the bundle. The strap is affixed to itself to provide tension to join the pipes. However, this mechanical strapping has the disadvantage that it depends on the application of adhesive to prevent slippage of the strap along a plastic pipe coating. This requires an additional step and does not provide a strong durable bond between the strap and the pipe.
In the second technique for bundling pipes, a large plastic clamshell is closed over the pipes. The clamshell is banded on with mechanical straps such as steel bands. Alternatively, the clamshell is bolted together. One or more bolts may be placed through the halves, approximately tangentially to the pipe. In another arrangement, one or more bolts may be placed down the axis of the pipe, passed through openings in the adjacent halves of the clamshell. The clamshell bundle jacket has the disadvantage that it is only mechanically attached to the pipe bundle and does not provide a strong durable bond between the clamshell and the pipe. In addition, this technique has the disadvantages of added cost and mechanical bulk.
It is therefore desired to provide a unified and versatile method and apparatus for applying a wrap to a pipe coated with a similar or compatible thermoplastic corrosion protective coating material that is capable of bonding to the yard-applied material. It is further desired to provide a system and device that allows a wrap to be fused to a pipe so that each of the following may be accomplished. An area of a weld joint may be coated in a manner that provides a seal of the same quality and physical integrity as the coating on the remainder of the pipeline. Similarly, an area of insulated pipe end may be coated in a manner that provides a seal of the same quality and physical integrity as the coating on the remainder of the pipeline. Further, more than one thermoplastic-coated pipe may be bundled by a jacket which is affixed such that it does not slip along the bundle. Still further, a ribbon anode in a pipe bundle may be prevented from sagging away from the bundle. Finally, an inner pipe may easily, inexpensively and effectively be held in place within an outer pipe. It is further desired to provide a method and article for accomplishing each of these tasks that is easy to use and that is not highly dependent upon the skill of manual labor for uniformity of results.